A magnetic field sensor for measuring a magnetic field is known from the German OS 2,921,546. This device utilizes a Hall element as the magnetic field sensor. In order to increase the sensitivity of the Hall element, said Hall element is fed not continuously but in impulses. The amplitude of the current fed by impulses can be selected to be greater than the amplitude of the highest admissible continuous current. It is limited by the criterion that the pulsating current integrated over time may not exceed the integral over time of the continuous current. The pulsating output voltage of the Hall element is amplified, rectified and fed to a measuring instrument.
A further device to measure a magnetic field is known (EP-OS 0,157,470) and is based on the generally known principle of the Forster probe. As with the Forster probe, an iron core is driven periodically into saturation. Instead of a measuring winding, however, a Hall element is used and is installed in a magnetic discontinuity of the iron core. If this device is subjected to a magnetic field, even-numbered harmonics are produced in the output signal just as with the Forster probe, and these are evaluated. The measured value is chopped up by the periodic saturation of the iron core. This results in a noise cancellation and an elimination of the offset of the Hall element. The excitation current must however be of sufficient magnitude since at least one magnetic discontinuity must be present to receive the Hall element.
When measuring lower, non-electric magnitudes over greater periods of time, the so-called 1/f noise and the offset of the sensor and amplification elements become very noticeable. A method which is known in measuring technology to eliminate the noise and the offset of DC amplifiers is the chopping method. The low direct voltage to be amplified is first converted by an interrupter circuit into a square wave voltage, the amplitude of which is equal to the input voltage. The resulting voltage is amplified in the manner of an alternating voltage. In alternating voltage amplifiers, shifts of the operating points remain without influence over a wide range, even if the amplification is increased to the limits of noise. The amplified square wave voltage is rectified and is supplied to a display instrument. This method is described in "Elektronik ohne Ballast" by Otto Limann (4th edition, page 139), Franzis-Verlag Munich, for example.
When magnetic fields are measured by means of magnetic field sensors using the DC method, the measuring precision is affected considerably by noise and offset of the sensors. Especially the lower measuring limit is increased by the 1/f noise and the measuring sensitivity is thus reduced accordingly.